1. Field of the Invention
The present invention relates to an apparatus for measuring the thickness of a plate-shaped article with a substantially rectangular lateral face.
2. Related Background Art
There is already known apparatus for measuring the thickness of an article, by moving said article in the direction of thickness thereof with respect to a measuring light emitted from a light emitting unit, and determining said thickness based on a signal from a photoreceptor positioned opposite to said light emitting unit and the amount of movement of said article. In such apparatus, said photoreceptor releases a signal showing different values according to whether the measuring light is intercepted or not by said article, and said apparatus measures the thickness of said article based on said signal and the amount of movement of said article.
However, such conventional thickness measuring apparatus may become incapable of accurate thickness measurement in case the article is composed of a translucent material such as a glass plate, since the measuring optical system and the article are so arranged that the lateral face of the article is perpendicular to the measuring light from the light emitting unit. In case of measuring the thickness of a translucent article, particularly having a flat lateral face, the output signal of the photoreceptor of said thickness measuring apparatus assumes a form as shown in FIGS. 7A and 7B. More specifically, said output signal, proportional to the amount of measuring light received by said photoreceptor, shows two bottoms B1, B2 because of the attenuation of the measuring light by scattering at both edges of the lateral face of the translucent article, and also shows a peak P since the measuring light is transmitted by said translucent article, almost without scattering in the non-edge part of the lateral face, and reaches the photoreceptor. If the thickness of the article is determined from the binary signal obtained by binary digitizing said output signal with a suitable slice level, there are erroneously given two thicknesses smaller than the actual thickness of the article. For this reason, the conventional thickness measuring apparatus has been incapable of accurate measurement of the thickness of a translucent article.